This invention relates generally to high-voltage power switches and more particularly to a high-voltage power switch that is actuated by blasting.
In power transmission and distribution systems, rapid current rises can occur, such as those occurring during short-circuit conditions. In order to protect the high-voltage power lines against the dynamic and thermal stresses which accompany the rapid current rises, the line must be electrically opened or cut-off before the short-circuit current has reached its peak value if the line is carrying alternating current, or before the line has reached its final value if the line is carrying direct current. The cut-off time required, which depends upon the frequency of the alternating current and on the inductance, capacitance and resistance of the power line, should not exceed a millisecond. Such rapid cut-off times, however, cannot be obtained with mechanically or magnetically actuated switches in medium-voltage and high-voltage networks. Therefore, switches have been developed which are actuated by blasting.
One conventional type of blast-actuated switch includes a hollow bridge conductor which electrically connects two external connections. A blasting cap is included in the hollow region of the conductor, approximately half way between each of the external connections. The blasting cap includes two wires which may be connected to an electric ignition device. The blasting cap is ignited, thereby blasting the conductor bridge with an explosive force. To prevent the scattering of fragments of the material which make up the bridge conductor at the time of blasting, the bridge is slotted in the longitudinal direction. Conductor webs, which are defined by the slots, each include a notch or a soldered joint at their longitudinal centers. Blasting the notches or soldered joints results in the webs being bent back to form rosettes around the associated external connection.
The energy stored within the inductance of the high-voltage power line will cause a step rise in voltage across the external connections when the bridge conductor is blasted. The voltage may rise to multiples of the operating voltage of the line. In order to prevent the rise in voltage across the separated ends of the bridge conductor from arcing and thereby delaying the cut-off process, a fusible wire is connected in parallel with the bridge conductor. The fusible wire is embedded in quenching sand. As soon as the rising voltage exceeds a predetermined value, the current flow through the fusible wire causes the wire to immediately melt, and the quenching sand prevents any further arcing. The fusible wire can be arranged and dimensioned so that the rise in voltage across the external connections is interrupted before the rising voltage reaches the breakdown potential of the gap between the separated ends of the bridge conductor. However, it has not been possible heretofore to prevent all arcing after the blasting of the bridge conductor in such conventional switches.
Arcing occurs in such conventional switches for primarily two reasons. First, the webs of the bridge conductor which are delineated by the slots have sharp edges. The edges become even more pronounced as a result of the blasting. After blasting, the density of the lines of flux and the gradient of the electrical field potential about the sharp edges will be very large in magnitude, thereby facilitating ionization of the gas contained within the switch casing. Second, the explosive force produced by the blasting caps used in such conventional switches is limited and therefore the axial extent to which the webs are bent back in rosette form by the explosion is also limited.
It is therefore an object of the present invention to provide a high-voltage, blast-actuated power switch in which the arcing which occurs in conventional switches after the bridge conductor has been blasted is more effectively prevented.